"Meet Claptrap" is an objective in the story mission Where Angels Fear to Tread.
Once in Thousand Cuts head into Slab Town. Cross over the Bridge to Bloody Knuckle Point and start to the South from there into No Man's Land. Remember, most Slabs are dumb and will just attack you because. Just head up the slope to Claptrap who is tagging part of the mountain. Talk with him to move onto the next objective.
Claptrap: "Minion! I've started my attack! I'm going to tag every inch of this mountain if it's the last thing I do. Let's go!"